Valorization and structural characterization of onion peel powder for the development of functional bread

ABSTRACT

Valorization of food waste has gained much popularity in current era due to the occurrence of various bioactive components in it. By-products obtained from the processing of onion are rich and dense source of bioactive components that can provide health-promoting effects when incorporated in various functional foods. Current study was conducted for the development of onion peel powder (OPP)-enriched bread. Moreover, the effect of onion waste supplementation on bread was further characterized structurally and functionally by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Overall, the mean results showed the addition of OPP significantly (p <0.05) modified the overall characteristics of functional bread as well as improved its antioxidant potential. The rheological properties determined through mixographic and farinographic analysis showed that the OPP added bread results in more stable dough having 64.67%, tolerance index 134.11% softness and while 6.32 min mixing time and 54.24 ± 0.08 BU peak height. The total phenolic contents (TPC) and flavonoid content (FC) of functional breads also increased while a visible increase in DPPH activity of functional breads was observed. However, SEM revealed the OPP-enriched bread had less fine protein structure because of the presence of higher fiber content as compared control sample while the FTIR analysis revealed the presence of various functional groups in OPP bread.

KEYWORDS:

• antioxidant profile functional bread
• FTIR
• onion peel powder
• rheological properties
• SEM

Introduction

Vegetables and fruits have gained much attention from the last few decades because they are rich in bioactive compounds and these bioactive compounds have many therapeutic potential, i.e. anti-carcinogenic effect, antioxidant potential as well as anti-mutagenic effect.^[1] Baysal and Ülkü^[2] documented that one-third of the annually produced food is wasted. The vegetable and fruit processing units produce a contrary of greater levels of waste (25–30% peels, pulp, seeds, shells, pomace, skins, cores, pods, etc.) than other food sector processing units.^[3,4] The onion (Allium cepa L.) is a plant that has long been utilized in numerous cuisines throughout the world. Its strong aroma and flavor is usually not liked by customers; however, its potential to boost human health makes it a valuable ingredient in many cuisines.^[5] The therapeutic properties of onion are due to the presence of quercetin, anthocyanins and other such components and characterized by its strong anti-glycation and antioxidant activities.^[6] Moreover, it has also been reported to provide strong antimicrobial^[7] as well as anti-inflammatory effects.^[8] The upper layer of the onion, which is usually dry, is removed during processing it as a waste but it contains higher biologically active components than other parts because it has less moisture content.^[9]

Onion (Allium cepa L.) production and cultivation are economic and commercial operations that are critical to the agro-food sector. Almost 99,968,016 tons of onion are produced globally from 5,192,651 hectares of area while almost 25% of quantitative and qualitative postharvest losses occurred annually.^[10] Increased onion processing has resulted in huge onion processing waste and the skin of onion contributes to maximum processing waste.^[11] The extract of onion waste has also documented to have hypocholesterolemic, antiasthmatic effect, enhance cardiovascular health and anticarcinogenic agent.^[12] The flavonoids present in the onion and onion waste also exhibit antiproliferative activity that have the ability to reduce excessive polyamine concentrations in body.^[13] Therefore, the incorporation of onion waste powder in bakery goods is a good source of energy and can be easily consumed by a wide range of population.^[14]

Bread is becoming an important food product in human diet throughout the world because of its century old processing history.^[15] People perceive food items not in terms of nutritional demands but also in terms of health enhancement. Currently, there is a rising interest in the development and consumption of functional foods. Functional foods are the foods that process biologically active components that provide health benefits to the host beyond basic nutrition.^[16] The fortification of food is one of the main techniques used nowadays for the development of desired functional foods.^[17] The bulk of the population consumes cereal items on a regular basis, such as bread. Wheat bread, in general, plays a significant function in the human diet. Increased customer demand for healthy bread has resulted in significant attempts to produce bread with both health advantages and high quality.^[18] According to the reports, addition of 3% onion skin waste can have positive effects on enhancing the antioxidant properties of bread.^[19]

A lot of research has been done for the preparation of bread and other baked goods with the incorporation of extracted polyphenols to improve the overall quality of final product and to solve the issue of the lack of nutrients in these products.^[20] The current study was planned to explore the potential utilization of onion waste for the production of value-added functional bread and to evaluate its structural characterization as well as rheological and antioxidant properties of the prepared bread.

Experimental study

Vegetable waste of onion was collected from domestic leftovers and small-scale local food industries of Faisalabad. The chemicals used for the research work were purchased from Merck (USA) and the research was carried out at different laboratories of Department of Food Science, Government College University Faisalabad, Pakistan.

Preparation of onion powder

Peels were washed with 0.5% chlorinated water to remove dirt and other impurities. Then, the peels were washed again using distilled water and left to dry in an open container. Afterwards, the dried peels were placed in oven drier at 70°C (Golden 2-0A) for 10 h and grounded to obtain fine powder using electric mill. The powder thus obtained was stored in food grade polythene packages and stored for further use.

Bread preparation

Functional bread was prepared by following the guidelines given in American Association of Cereal Chemists (AACC, 2000), following the method No. 10–10 with some desirable modifications.^[21] 95% straight grade flour was used with the addition of 3% of vegetables waste extracts obtained from UA-extraction technique. Four bread treatments were prepared which were denoted as B_c (control bread treatment) and bread-enriched with onion peel powder (BOPP).

Dough rheological properties

Before the preparation of bread, the dough formed with added extracts was evaluated by farinographic analysis for the determination of dough development time, water absorption time, mixing tolerance index, dough consistency, and dough softness, using the Farinograph according to method no 54–21.01 of AACC (2000). Mixographic analysis was conducted to analyze the peak height of the dough and mixing time using mixograph by adopting the AACC method no. 54–40.02.^[21]

Structural characterization of bread

Scanning Electron Microscopy (SEM)

Bread enriched with OPP was characterized on morphological basis using scanning electron miscroscopy (Emcraft cubeseries, South-Korea) available at Department of Physics, Government College University Faisalabad (GCUF) by following the protocol proposed by Medina-Jaramillo et al.^[22] Samples to be analyzed were fixed on the stubs and 20 kV was used to observe the samples.

Fourier Transform Infrared Spectroscopy (FTIR)

Onion peel waste-enriched bread was further characterized using FTIR spectrometer (Waltham, MA, USA) for the detection of various functional groups present in it by following the protocol of Kumar et al.^[23] The peaks were detected taking 60 scans per experiment using a resolution of 4 cm⁻¹ and transmission mode of 4000 to 600 cm⁻¹ was used in each experiment.

Phenolic and antioxidant activity of functional bread

The total phenolic content and flavonoid content (FC) of the bread treatments were measured by following the method of Jiménez-Moreno et al., 2020, with some minor modifications.^[24] Folin Ciocalteu reagent (1000 μl) was added in 200 μl of each extract sample and after 5 min, 800 μL of 7.5% sodium carbonate was added in each solution and solution were kept in dark for 1.5 h. Afterwards, the absorbance of each sample was measured at 765 nm using spectrophotometer (UVI720, China). The concentration of TPC was measured using calibration curve of Gallic acid at different concentrations regression equation. The readings were taken and represented as micrograms of gallic acid equivalent per milligram of dry extract (mg of GAE/g of extract).

For the determination of total flavonoids, 250 μl of each extract sample was added in a beaker with same volume of aluminum tri-chloride (AlCl₃) having 2% ethanol in it. The samples were mixed for 8–10 min on hot plate and the absorbance was measured at 430 nm using spectrophotometer (UVI720, China). Calibration curve of quercetin was used for the quantification of flavonoids, and readings were noted in mg of QE/g of extract.

However, DPPH activity was accessed by following the method of Sir Elkhatim et al. (2018), with some modification.^[25] While performing DPPH activity was evaluated using 3 ml of each sample with 0.1 M DPPH-ethanol solution (2 ml) with 1 ml of HCl buffer solution. After 30 min, absorbance was detected at 517 nm and DPPH activity was noted as:

$DPPHactivity\left(\mathrm{\%}\right)=\left[\frac{\mathrm{A}\left(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}\right)-\mathrm{A}\left(\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\right)}{\mathrm{A}\left(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}\right)}\right]\times 100$

Statistical analysis

All the experiments were conducted in triplicates and the collected data were subjected to Statistix 8.1. Analysis of variance was applied on the data and a multiple comparison test was performed for all the means, using least significant difference at 0.05 levels.

Results and discussions

Farinographic and mixographic characteristics

When wheat flour and vegetable waste extracted powders were mixed together with water, a wet mass of dough was formed. Because of the intricate interactions between wheat ingredients, the dough was developed during the mixing process. The dough’s development begins with the addition of water and continues with the mixing process. The amount of water that flour absorbs to retain the appropriate consistency and generate a superior dough for bread making is known as water absorption. It is the amount of water that the flour required to form a wet mass to form dough with optimal stability for bread manufacturing without becoming too sticky. Table 1 showed the farinographic results of all the treatments (Do and DOPP). Water absorption studies demonstrate that Do dough absorbs the least amount of water (60.17 ± 1.25%), while enriched flour dough with onion powder, DOPP, absorbs more water (64.32 ± 1.90%). These results are significantly (p < .05) different from each other. Functional breads with added extract powders had higher swelling strength property and water retention potential than wheat flour, and the results suggest that better water absorption could mitigate the un-favorable impact on gluten network formation and bread quality. In protein and starch molecules, the hydration mechanism is performed by creating hydrophilic interactions with water molecules along with hydrogen bonds.

Table 1. Farinographic and mixographic profile of dough treatments Do (Control treatment) and DOPP (bread prepared with onion peel powder). The results are presented as mean ± standard deviation. The results differ from each other by least significant difference in lowercase letters.

Farinograhic Characteristics					Mixograhic Characteristics
Treatment samples	Water absorption (%)	Dough development time (min)	Tolerance index (%)	Softness of dough (%)	Mixing time (min)	Peak height (BU)
Do	60.17 ± 1.25^b	6.63 ± 0.18^b	62.83 ± 1.02^b	133.37 ± 1.18^a	6.23 ± 0.01^b	55.44 ± 1.03^a
DOPP	64.32 ± 1.90^a	7.15 ± 0.03^a	64.67 ± 0.31^a	134.11 ± 2.49^b	6.32 ± 0.02^a	54.24 ± 0.08^b

Dough mixing is a process that involves mixing of the flour in water until gluten network is created in the dough and resulted on increased interaction between distributed and hydrated gluten-forming proteins. The time required for the development of dough was higher DOPP (7.15 ± 0.03 min) than control treatment (6.63 ± 0.18 min). The possible reason might be that the powder have more fiber content that possibly enhanced dough surface area. The goal of the dough development process in particular was to make bread dough, having some physical changes in dough characteristics and to improve its capacity to absorb CO₂ gas that emits during fermentation. Dough stability refers to the point at which dough over mixes due to the breakdown of gluten network of dough. Mixing is an important and critical phase, and it is impacted by the mixer speed, dough temperature, flour water absorption and the amount of shortening in the dough recipe. The mixing tolerance index was 62.83 ± 1.02% in Do and 64.67 ± 0.31% in DOPP, respectively. From these results, it can be noted that the mixing tolerance index with the addition of peel powder increases.

A slight reducing trend was observed for the softness of dough with the addition of vegetable waste powder. The results revealed that the softness of dough was highest in Do the softness was observed 134.3 ± 1.18%, while the values observed for DOPP was 133.04 ± 1.02%, respectively. A study conducted by Han and coworker in 2021 reported that the addition of peels of agro-waste not only improves the quality of bread when added in it but also improves the overall bread’s rheological properties.^[26]

The mixographic analysis of the prepared dough reveals that lower time was required for the development of control dough. However, its peak height was higher than the functional dough. The lower the time is required for the preparation of dough, more soft and more elastic it will be and higher the time required for the development of dough, less soft and less elastic dough it will be. A study conducted by Han et al.^[26] reported that the addition of agro-waste, i.e. peels, improves the quality of the resultant bread with improved flavor and overall total polyphenols and fiber of the bread.

SEM observation

The results for scanning electron microscopy (SEM) of bread samples are shown in Figure 1. SEM images revealed that both bread had different surfaces. It means that the samples had different pore sizes and different holes density and integrity. The control bread sample discloses more irregular holes having different sizes, shape, and density on the surface; however, the structure of the bread was stable. OPP-enriched bread showed a denser structure with more irregular-shaped holes structure which are smaller in size than the control sample. This difference can be observed from the images that the particles of bread enriched with the OPP had less swollen structure of granules due to limited gelatinization process because it contained more fiber than the control bread that decreases the absorption of water for the development of higher levels of gluten network. However, on the other hand, the control bread showed swollen network because it contained more gluten network and resulted in more tender bread with more uniform structure of proteins interwoven with the starch granules. However, the OPP-enriched bread did not have the fine protein structure with starch granules because of the presence of higher fiber content that reduces the water availability for the development of proper gluten network as in control sample. These fibers engulf or overlapped the structure of dough and cause the development of more holes that may weaken the structure of enriched bread with OPP. Similar results for the development of bread with onion waste were also reported by Siddiqui et al. (2022); they reported that the presence of higher levels of fiber content in the bread from onion skin powder competes with the starch and results in lower gluten protein network in bread.^[27]

Figure 1. Scanning electron microscopy (SEM) images of bread samples. (a) control bread treatment, (b) bread enriched with OPP.

Figure 2. FTIR spectra of analysis of bread samples. (a) FTIR spectra for control bread treatment, (b) FTIR spectra for bread treatment enriched with OPP.

FTIR spectra analysis

Fourier transform infrared (FTIR) spectra obtained for bread containing OPP are shown in Figure 2. The obtained spectra showed various distinctive peaks at various intervals that are the indication of the presence of various functional groups. The peaks appeared between 1600 cm⁻¹ and 600 cm⁻¹ revealed the presence of biomolecules (polyphenols, lipids, proteins, and carbohydrates). Lu et al.^[28] also reported the presence of bioactive components at the similar range during FTIR analysis. Medium narrow peaks can be observed from the graphs at 3500 cm⁻¹ to 3800 cm⁻¹ indicating the presence of medium and strong O-H (alcohol) stretching groups. A study by Rasheed et al. (2023) also indicated the presence of similar O-H stretching groups.^[29] However, according to Wellner et al., (2013) the absorption at such wavelengths (3500 cm⁻¹ to 3800 cm⁻¹) due to the enhanced intensities of C – H stretch aromatic groups and O – H obtained as a result of interaction of polyphenols added in the extracts or added powders form plant source (i.e. quercetin in onion) and bread molecules.^[30] However, in control bread treatment, graph showed a wide range of low and narrow peaks. Weak peaks were observed in both bread treatments at wavelength 3000 cm⁻¹ that showed the presence of strong N-H stretching groups, which indicates the presence of amine salts. Narrow peaks were obtained at 2366 cm⁻¹ in control bread treatment while sharp peaks were obtained at 2369 cm⁻¹ and indicate the presence of strong O=C=O stretching groups. Medium peaks falls between the frequency range of 1600 cm⁻¹ − 1700 cm⁻¹ that reflects the presence of strong C=O stretching conjugated acids and primary amides and C-F stretch aliphatic organic halogen compounds and aromatic nitro compounds. However, the control bread treatment at 1541 cm⁻¹ wavelength showed the presence of strong N-O stretching nitro compound and along with conjugated acids. Weak C-H bending groups were also present at 1400–1450 cm⁻¹ wavelength, reflecting the presence of alkane from methyl group in both breads treatments. Medium peak frequencies were also observed at 600 cm⁻¹ to 1000 cm⁻¹ reflecting the existence of C≡C stretch alkene groups, strong C-Br stretching halo groups; however, the peaks were overlapping at a frequency range of 600 cm⁻¹.

Antioxidant and phenolic profile of bread

Figure 3 shows the total phenolic, flavonoid content, and DPPH activity of bread treatments. Results showed that bread with added OPP showed higher antioxidant properties than control treatment. Higher DPPH activity was observed for bread enriched with OPP treatment. It may conclude that the overall antioxidant potential increased by adding extract powders. DPPH activity increases almost two times by the addition of waste powders. Similarly, the phenolic contents and flavonoid contents were noted maximum in BOPP, i.e. bread prepared with onion waste/peel powder. The lowest phenolic contents were observed in control bread treatment that indicates the phenolic content increase almost 2–3 times in functional bread treatments. However, Bourekoua et al. (2018) also presented the antioxidant potential of bread and their studies are in accordance with the findings of our study.^[31] A similar study was conducted by Baiano et al. in 2015, they utilized the waste extracts of different vegetables and documented that the phenolic contents were associated in functional breads by the interactions between the protein present in bread and phenolic antioxidants due to the presence of hydrogen bonding in the bread dough.^[32]

Figure 3. Total phenolic content, flavonoid content and DPPH activity of bread treatments. Bc (control bread), BOPP (bread enriched with onion peel powder).

Conclusion

The finding of the current study suggests that onion waste is a rich source functional ingredients including phenolic compounds and antioxidants that can be utilized for functional food product development that is capable to enhance the overall nutritional and antioxidant potential of the food products. The rheological properties of the dough prepared with the onion waste reveal the overall softness if the dough increased. Addition of OPP significantly improves bread quality structurally and functionally with enhanced DPPH activity, total phenols and flavonoids of the functional bread.

Acknowledgments

The authors are thankful to Department of Food Sciences, Government College University Faisalabad and for providing technical support and laboratory facilities during research work.

Disclosure statement

No potential conflict of interest was reported by the author(s).